Radar sensors are used in motor vehicles in conjunction with driver assistance systems, for example, distance warning systems and regulating systems, for location of objects, in particular other vehicles in the surroundings of the host vehicle. For example, long-range radar (LRR) sensors, which operate at a frequency of approximately 77 GHz as well as short-range radar sensors (SRR) at a frequency of 24 GHz are customary. If the radar sensor is installed in the vehicle and functions to measure the distance to preceding vehicles, the lens has the purpose of bundling the emitted and/or received radar radiation to form at least one radar lobe directed forward, so that the maximum intensity and sensitivity are achieved in the angle range in which preceding vehicles are normally located, whereas objects farther away from one's lane cause little or no radar echo. In the case of an angular resolution radar sensor, a plurality of radar lobes fanning out in azimuth is generated so that the azimuth angle of the object having been located may be deduced from amplitude and phase ratios between the signals obtained from the different lobes.
In the case of a customary design of radar sensors for motor vehicles, a spherical lens or, more generally, a lens in the form of a rotational body is used, this lens being made of a plastic having a high refractive index for the particular frequency of the radar radiation and bundling the radiation like a converging lens. Essentially the same directional characteristics are achieved in azimuth (in the horizontal) and in elevation (in the vertical). However, in the case of angular resolution radar sensors having a plurality of antenna elements situated side by side in the focal plane of the lens, a certain modification of the directional characteristic in azimuth may occur due to interference between these antenna elements.
Frequently, however, in particular in the case of long-range radar sensors, it would be desirable to bundle the radiation in elevation to a greater extent than in azimuth, so that on the one hand a sufficiently wide field of vision is achieved in azimuth, but on the other hand, due to stronger bundling in elevation, unnecessary power losses are avoided and at the same time interference signals due to reflection from the road surface (ground clutter) or the like are suppressed better. One possible method of achieving such an anisotropic directional characteristic, i.e., differing in elevation and in azimuth, is to use complicated lens systems having a plurality of lenses. In the case of radar sensors for motor vehicles, however, this is not practical for reasons of cost and because of the great amount of space required for the lens systems.
German Patent No. DE 10 2007 036 262 describes a radar sensor whose lens is composed of two plane-convex cylindrical lenses situated back to back, their cylinder axes running at right angles to one another and having different focal distances.